1. Field of the Invention
The present invention relates to a particle detector for detecting particles contained in fluid in which the fluid to be scanned is introduced into a particle detecting region defined by radiated laser light.
2. Description of the Prior Art
At a manufacturing site for precise electronic devices, higher decontamination has been required for areas such as a clean room. It is necessary to pass a great volume of sample fluid through a flow path of a particle detector at one time so as to detect particles in a highly decontaminated condition. Also, it is necessary to increase a cross-sectional area of a flow path so as to pass a great volume of sample fluid, and as a result of increasing a cross-sectional area of a flow path, it is necessary to expand a laser beam so as to secure a particle detecting region.
As disclosed in Japanese Unexamined Patent Application Publication No. Sho 59-104533, there has been already known a conventional particle detector in which a flow rate of a sample is increased by employing a multimode laser using a plasma tube.
However, the transverse mode pattern of such a multimode laser using a plasma tube, such as a He-Ne gas laser, has a circular shape because the plasma tube is comprised of a capillary glass tube having a circular cross section.
Accordingly, if a laser beam having a circular cross section is expanded, the energy density thereof is deteriorated, an amount of light scattered by particles is decreased, and thereby it becomes difficult to detect fine particles. Also, since the plasma tube is comprised of glass, it is inferior in the thermal and mechanical strength, optical axis displacement may occur, and there is a strong likelihood that the accuracy of detecting particles will be deteriorated. In addition, the particle detector is large-sized because of using a plasma tube, and thereby it is not user-friendly.
As a particle detector using a solid-state laser for miniaturization, there has been known a particle detector in which a laser beam has a single transverse mode such as disclosed in U.S. Pat. No. 5,726,753. However, it is impossible to expand such laser beam of a single mode.
In a conventional particle detector shown in FIG. 4, the beam thickness having a single mode (the beam diameter at the end surface of a solid-state laser 102) xcfx89 is obtained by the following equation (1):                     ω        =                  2          ⁢                                    (                                                                    (                                          λ                      π                                        )                                    2                                ⁢                                  L                  ⁡                                      (                                          R                      -                      L                                        )                                                              )                                      1              4                                                          [                  Equation          ⁢                      xe2x80x83                    ⁢          1                ]            
In this equation, xcex indicates the wavelength of laser light La within an optical cavity 100, L indicates the length of the optical cavity 100, and R indicates the radius of curvature of a reflecting mirror 101. Also, the reference numeral 103 refers to a semiconductor laser which functions as a light source for optical pumping, and the reference numeral 104 refers to a condenser lens. FIG. 5 is a graph showing the intensity distribution of a laser beam in a single mode.
As is apparent from Equation 1, for a practical optical cavity length, it is difficult to achieve a beam diameter of 1 mm or more. Therefore, if a great volume of sample fluid (for example, 28.3 liter/min.) is to be passed through, it is necessary to increase the flow velocity. As a result, light interference occurs due to the flow of the sample fluid, which causes a noise increase, and thereby it is difficult to detect fine particles.
It is therefore an object of the present invention to provide a particle detector which allows a great volume of sample fluid to pass through a flow path at one time and thereby achieve accurate monitoring in high decontamination conditions.
For solving the above-mentioned problems, according to the present invention, there is provided a particle detector comprising an optical cavity constructed of a solid-state laser which is optically pumped by pumping light generated from a pumping light source and a reflecting mirror, the optical cavity generating laser light, a flow path defined by sample fluid, and a particle detecting region where the laser light is radiated upon the flow path, wherein particles passing through the particle detecting region are detected by receiving light scattered from the laser light due to the particles passing through the particle detecting region, and wherein the laser light generated within the optical cavity has a multi transverse mode.
According to another aspect of the present invention, the cross sectional shape of the laser light has a different length in the longitudinal direction and the transverse direction.